With the recent advancement in electronic technology, portable electronic devices, such as cell phones, portable personal computers, personal data assistances (PDAs), and portable game machines, have rapidly become popular. There is accordingly an increased demand for power storage devices, such as secondary batteries that can be repeatedly charged and discharged, as the power source for portable electronic devices. Among them, lithium ion secondary batteries are widely used as the power source for portable electronic devices, since they have high electromotive force and high energy density and can be made more compact relatively easily.
In order to make portable electronic devices versatile, portable electronic devices are required to provide improved performance. For example, they are required to be more light-weight, more compact, and more multifunctional. Accordingly, batteries used as the power source for such portable electronic devices are required to provide higher energy density and the like. An effective approach to heightening the energy density of a battery is to use an electrode active material having a high energy density. Therefore, new materials having high energy densities are actively studied and developed for both positive and negative electrode active materials.
For example, the use of an organic compound capable of reversible oxidation-reduction reaction as an electrode active material is examined. An organic compound has a specific gravity of approximately 1 g/cm3 and is more light-weight than an inorganic oxide such as lithium cobalt oxide, which has been commonly used as an electrode active material. Hence, the use of an organic compound as an electrode active material is thought to provide a power storage device with a high weight-energy density. Also, since no heavy metal is used as an electrode active material, it is possible to reduce risks such as depletion of rare metals, price fluctuations due to such depletion, and environmental pollution by leakage of heavy metals.
In one example of research and development on the use of an organic compound as an electrode active material, the use of a quinone organic compound as an electrode active material in combination with an aqueous solution type electrolyte has been proposed (e.g., see Patent Documents 1 to 3). Quinone organic compounds generally refer to compounds having an aromatic ring and two ketone moieties (C═O) bonded to the aromatic ring, and derivatives thereof.
Patent Document 1 proposes using an organic compound having two ketone moieties (C═O) at the para-positions of an aromatic ring (hereinafter referred to as “paraquinone compound”) or an organic compound having two ketone moieties at the ortho-positions of an aromatic ring (hereinafter referred to as “orthoquinone compound”) as an electrode active material for an aqueous solution type secondary battery. Patent Documents 2 and 3 propose using a polymer of a paraquinone compound as an electrode active material for an aqueous solution type secondary battery.
In the secondary batteries of Patent Documents 1 to 3, the charge and discharge reactions are effected by addition (C—OH) and elimination of a proton to and from the ketone moieties (C═O). These Patent Documents state that the use of a quinone organic compound as an electrode active material can provide a secondary battery having good reversibility, i.e., good charge/discharge cycle characteristics. However, these aqueous solution type secondary batteries usually have low battery voltages of approximately 1 to 2 V, thus being inferior to lithium ion secondary batteries having battery voltages of approximately 3 to 4 V. It is therefore difficult to obtain a power storage device whose energy density is as high as that of lithium ion secondary batteries.
The use of a quinone organic compound as an electrode active material is also examined for non-aqueous type power storage devices, which are expected to have higher energy densities than aqueous solution type secondary batteries. For example, the use of a quinone organic compound as an electrode active material for a power storage device using an electrolyte comprising an organic solvent and a lithium salt dissolved therein has been proposed (e.g., see Patent Documents 4 and 5). Patent Document 4 uses 9,10-phenanthrene quinone, which is an orthoquinone compound, as an electrode active material. Patent Document 5 uses a polymer of a paraquinone compound as an electrode active material.
Also, it has been proposed to use a quinone organic compound having quinone moieties at the ortho-positions or para-positions of an aromatic ring, such as a rhodizonic acid salt, a rufigallic acid salt, an oxidation compound thereof, ellagic acid, or an oxidation compound thereof, as an electrode active material for a lithium ion secondary battery (e.g., see Patent Document 6).
However, it is difficult to commercialize conventional non-aqueous type power storage devices using quinone organic compounds, since the reversibility is insufficient and the discharge voltage is low, compared with conventional power storage devices using inorganic oxides such as lithium cobalt oxide.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 4-87258    Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 11-126610    Patent Document 3: Japanese Laid-Open Patent Publication No. 2000-40527    Patent Document 4: Japanese Laid-Open Patent Publication No. Sho 56-86466    Patent Document 5: Japanese Laid-Open Patent Publication No. Hei 10-154512    Patent Document 6: Japanese Laid-Open Patent Publication No. 2001-512526